Retinoic acid inducible-gene I (RIG-I) is a cytosolic multidomain protein that detects viral RNA and elicits an antiviral immune response. Two N-terminal caspase activation and recruitment domains (CARDs) transmit the signal, and the regulatory domain prevents signaling in the absence of viral RNA. 5′-triphosphate and double-stranded RNA (dsRNA) are two molecular patterns that enable RIG-I to discriminate pathogenic from self-RNA. However, the function of the DExH box helicase domain that is also required for activity is less clear. Using single-molecule protein-induced fluorescence enhancement, we discovered a robust adenosine 5′-triphosphate-powered dsRNA translocation activity of RIG-I. The CARDs dramatically suppress translocation in the absence of 5′-triphosphate, and the activation by 5′-triphosphate triggers RIG-I to translocate preferentially on dsRNA in cis. This functional integration of two RNA molecular patterns may provide a means to specifically sense and counteract replicating viruses.
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